As is well known, a synchronous transmission system (STS) is widely utilized to accommodate as many voice/data channels as possible in a given communications path. Specifically, the STS converts asynchronous frame data such as digital signal level 1 (DS-1) of the North America or digital signal level 1 (DS-1E) of Europe into synchronous frame data of, e.g., synchronous transport module level 1 (STM-1), wherein there is employed a set of overhead data. The DS-1 and the DS-1E data represent pulse code modulation (PCM) serial data of a first transfer rate, e.g., 1.544 Mbps, and PCM serial data of a second transfer rate, e.g., 2.048 Mbps, respectively, whereas the STM-1 data represents PCM parallel data of a third transfer rate, e.g., 155.5 Mbps. Such a conversion operation is typically carried out on the basis of a synchronous digital hierarchy (SDH) issued by International Standards Union-Telecommunications Standardization Sector (ITU-TS).
In order to efficiently perform the conversion operation, there are generally provided two tributary unit group (TUG) clock signals in the STS, wherein the two TUG clock signals are represented by a predetermined identical clock frequency, e.g., 6 Mhz, and one of them is used as a system main clock signal. When a TUG clock signal, selected from the two TUG clock signals to be used in the STS, is detected to be erroneous or in failure, the STS immediately replaces the erroneous TUG clock signal with the other to continue the conversion process by using the replaced TUG clock signal.
As is well known in the art, such an error detection operation for each of the two TUG clock signals is performed by using a conventional clock signal testing method. In the conventional clock signal testing method, two multivibrators, and several resistors and capacitors coupled with each of the multivibrators are generally employed to test whether each TUG clock signal is normal or not. That is, each TUG clock signal is considered to be erroneous if it is not supplied to its corresponding multivibrator for a predetermined time duration, wherein the time duration depends on a time constant which is proportional to a total impedance value of the resistors and the capacitors connected to the corresponding multivibrator.
Since, however, the conventional clock signal testing method has to employ analog circuitries such as resistors and capacitors to test each of the two TUG clock signals, it is rather difficult to precisely adjust the circuit parameters to obtain the predetermined time duration employed in testing each TUG clock signal, thereby degrading the performance of the STS.